Synchroballistic, photofinish, panoramic, peripheral, and certain reconnaissance
camera systems all share a common bond in that they are variations on a
theme dependent on imaging systems in which the subject images and
the recording medium both move past a stationary slit "shutter". The system
in general is sometimes referred to a "strip" photography.

In conventional photographs of high speed projectiles in flight one
constantly struggles to reduce the exposure time to a low enough value
which will yield images of acceptable sharpness. Since the resolution of
fine detail in a subject is a function of the relationship between image
motion with respect to the film and the exposure tine, large image velocities
with respect to the film call for correspondingly short exposure times
given the desire to resolve a particular detail in the object.

One way to minimize image motion is to "pan" the camera with the moving
object. This allows longer exposure times than if the subject simply moved
at right angles accross the field of view of the stationary camera. Yet
another method is to move the film slightly in the direction of image motion
during the time that the subject is photographed. Both of these methods
generally depend on accurate synchronization between the exposure and the
presence of the event within the field of view of the camera.

Synchroballistic photography overcomes both the problem of synchronization
and exposure time by making the image of the object pass over an open slit
while at the same time the recording medium passes below the slit at approximately
(or exactly if possible) speed as the image. Since in this manner the image
does not move with respect to the film a sharp record can be secured even
at relatively long exposure times, these being a function of the width
of the slit and the rate of film movement.

As in other applications, in synchroballistic photography it is also
useful to know the rate at which an object will be moving at right angles
to the optical axis. Then, the Film Velocity which must be set into the
camera is simply the Object Velocity divided by the Optical Reduction of
the camera system. Exposure Time will be the Slit Width divided by the
Film Velocity. This allows one to calculate the required F# to be used
given a particular film and lighting combination.

Or, a test can be simply run at some apparently logical Film Velocity.
Upon analyzing the image the appropriate correction to the previous speed
can be made by examining the record for length vs. widthwise distortion
compared to the ratio of the subject9s length vs. width, If the record
appears stretched out, let9s say by a factor of about 2 times vs. the original9s
proportions then the film velocity needs to be reduced by a factor of two.

Conversely, if the record appears compressed, this is an indication
that the film was movino too slowly and its velocity needs to be increased
for the next run. By the way, the film must move within the camera in the
opposite direction than that in which the subject moves so that the image
of the subject and the film will move in the same direction,

Regardless of the apparent distortion, however, a synchroballistic record
is very often an excellent means for determining the velocity, attitude
and rotation rate of a projectile. For this to be possible some precautions
need to be taken in terms of the placement of markings of known dimensions
and orientation on the test missile.

For example, the average velocity of a missile over a distance equal
to its length can be computed if the Film Velocity is known. It is equal
to the length of the real missile divided by the value of the length of
its image divided by the Film Velocity. The units for the missile length
may be different than the units for image length and film velocity but
these two must be in the same units of length.

By taking the trouble of incorporating a straight line extending from
the missile tip to its tail, rotation rate can also often be determined.
This is done by analyzing the image in terms of small segments extending
from tip to tail and noting the position of the Iine with respect to the
edges of the missile body.

If the line invariably
appears centered between the edges of the image of the rocket and if the
original line was in fact straight, then the assumption which can be made
is that there was no rotation as the missile's image moved from tip to
tail accross the slit of the strip camera. However, if the mark appears
to move from one edge of the missile to the other as one examines the record from tip
to tail, then rotation took place. The rotation rate is simply a function of the
number of degrees of displacement which would cause the noticed movement
of the line from one edge to the other divided by the number which results
from dividing the distance over which the movement is measured by the film
velocity.

Further, adding lines at right angles to the above line, changes in
the attitude of the missile can also be estimated by noting the degree
to which these line are no longer paralell to each other and the distance
( or time ) over which this effect is measured. In addition vibration of
the missile can sometimes also be detected and measured. The fact that
the synchroballistic record not only delivers a sharp record without major
synchronization problems is of itself noteworthy. That it can also deliver
so much additional quantitative information is probably simply amazing!

In this demonstration set-up you are invited to make your own synchroballistic
photograph by simply attaching the aluminum film grabber clamp onto the
white tab extending from the side of the Polaroid camera. Use the allen
wrench to make a firm connection. Turn the power to the camera ON briefly
to take up the slack in the steel cable then turn camera OFF.

Now, turn
the missile sled power pack ON if the sled is not already running. Wait
for it to move as far from the camera as possible and when it starts its return trip
turn the power to the
camera ON once more and wait for the white tab to completely exit the side
of the camera. Turn the power to the camera OFF at this time.

Pull out
the dark film processing tab from the camera side trying not to disturb
the alignment of the camera. Wait 40 seconds and them peel the print from
the negative. If you took the trouble to time the length of time it took
the film to go through the camera you can figure out the Film Velocity
since one exposure is equal to about 4 inches. Assuming that it took 10
seconds from the start to the finish of the exposure your film moved at
4 inches divided by 10 seconds or .4 inches per second.

If the real rocket
measures 8 inches and its record measures let's say 2 inches, then the
rocket traveled a distance equal to its real length or 8 inches in 2 inches
divided by .4 inches/second or 5 seconds which then results in a missile speed of 1.6 inches
per second.

If you have any questions about this demonstration or synchroballistic
photography in general contact Prof. Andrew Davidhazy, Rochester Institute
of Technology, Imaging and Photographic Technology, 70 Lomb Memorial Drive, Rochester,
NY 148Z3 (585)475-2592 or send email to him at: andpph@rit.edu